Hi there,
You can run a Customer Balance Detail/Summary report in QuickBooks Online to view the Accounts Receivable open balance for your sub-customers or projects. While there isn't a report that lists projects with a balance due, the customer balance report offers a detailed breakdown of open accounts receivable.
Head to the reports tab and search for Customer Balance Detail report. Choose if you want to view the detailed or summarized version of the report.

After that, you can also consider customizing the report and saving it so you can access it easily in the future.
Feel free to leave a reply if you have additional concerns in QBO. We're always ready to help.